CamH: I 100% agree, but trying to convince someone who's been a Telecom customer for 30+ years to move is near impossible. I just think it's ridiculous in such a technology focused age where this sort of stuff is super important that nobody can do a 30 second nameserver change on a weekend.

Surely this has just convinced them?

You would think so wouldn't you? Unfortunately not!

They sound like they are going to learn the hard way. Especially as nameserver changes can take many hours to propagate across the internet.

Name server changes don't "propagate". Servers retrieve a record with a TTL (Time To Live). *IF* these servers are properly configured they will retrieve fresh records at the end of TTL. If your domain has a high TTL then it will take longer for servers to retrieve fresh records when queried. On the other hand if the TTL is too low you put too much load on NS servers.

Some servers are badly configured and ignore TTL, only retrieving records every few hours.

But DNS does not "propagate".